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The current trend in private sector communications is toward supporting all IP-based broadband multimedia applications. One need only witness the mad rush to buy Android-based devices to appreciate this phenomenon. This trend is the result of significant technology advancements in the underlying communications core networks and the mobile devices used by subscribers. In contrast, today’s mission critical public safety communications are primarily voice-only applications and only now are gradually migrating from analog towards digital trunk-based standard network operations. This migration improves channel utilization and interoperability, but the supported applications remain mostly narrowband voice centric services. Furthermore, spectrum has been allocated for public safety essentially for narrowband services over five major frequency bands in Low Band VHF, High Band VHF and UHF at 700 MHz and 800 MHz. This fragmented spectrum allocation along with the historically site-specific and agency-specific nature of licensing public safety spectrum, lack of governance, small purchasing power of the public safety community and the nature of the public safety vendor community have hindered interoperability and have not allowed economies of scale for user equipment in the same way that private services have expanded.

Today’s channel allocations in public safety communications bands can handle limited data applications, but emerging applications demand higher data rates and broadband capabilities for communications among first responders and public safety agencies. First responders in emergencies are beginning to recognize video applications and visualized location-based services as mission critical. Such services include applications to exchange data, graphics, and video information that certainly require public safety communications capable of multimedia broadband operations. The term broadband commonly refers to high-speed Internet access. However, the forthcoming multimedia and data applications require much higher rates than those currently available. Disasters and other incidents are simply requiring reliable communications paths with higher bandwidth requirements. The focus of this Tech Topic will be on broadband networks and next generation cellular capabilities to support public safety broadband requirements.

The Safecom Program1, along with many other resources within the DHS, is an example of numerous programs that have identified applications requiring broadband capabilities that may be useful to emergency responders. For instance, some mission-critical applications require high data rates for broadband operation such as real-time high-quality video from a remote scene and exchanging diagrams or blueprints of building layouts between dispatchers and officers on the scene. Some of the non mission-critical applications can be done over multi-purpose public/municipal networks operating in license-exempt bands or through commercial wireless services. On the other hand, numerous articles have suggested that public safety communications in the future will require broadband connections that accommodate data, still image and video (multimedia) in addition to the basic mission-critical voice application.

The current generation of public safety communications incorporates trunked digital narrowband standards technology such as those outlined in Project 252, but still rely heavily on voice communications using circuit-switched based network architectures. Even as the public safety standards community is working on broadband communications through programs such as Project MESA3, progress is far slower than the advancement within the commercial sector. Mobile wireless technology has advanced dramatically for commercial and military systems, but this has not been the case in public safety systems. In addition, many public safety agencies nationwide must respond cooperatively to large-scale disasters whether man-made or due to natural causes. This may require a single common communications system and even communications connecting to private sector broadband networks that hold critical information to help in rescue and relief missions.

All of these issues suggest that public safety communications could benefit by adopting standards-based commercial broadband wireless technologies. Thus, the development of next generation networks (NGN) for public safety communications should encompass networks that will be broadband, Internet Protocol (IP)-based and capable of handling multimedia content including voice, data, images and video. Further, and perhaps as important as the communications requirements, is the cost of capital investment for public safety to build a reliable and ubiquitous network. Clearly, public safety could benefit from using the capabilities of next generation commercial cellular systems and as a result one of the simplest approaches may be for emergency responders to adopt commercial wireless type services for their communications networks, as many agencies do today.

The FCC has envisioned such broadband services for public safety and the mutual benefits of utilizing a common nationwide network infrastructure via a public/private partnership4. The prior allocation of spectrum for dedicated broadband use in the 700 MHz public safety bands could promote a combined infrastructure for public safety and commercial communications as part of spectrum reallocations of the former 700 MHz TV band. Hence, the FCC has taken an important step towards the development of a nationwide interoperable broadband public safety communications network.

In order to achieve these goals for public safety communications services, it is essential for overall operational performance requirements to adopt the appropriate network technology. The emerging Long Term Evolution (LTE) wireless technology is one standard that has promise for public safety broadband communications as well as providing the evolutionary path for next generation cellular services. The Third Generation Partnership Project (3GPP5), which was formed in 2004, has been developing the LTE standard. Furthermore, the 3GPP development efforts are being expanded beyond today’s 3G Wireless requirements in order to meet ‘IMT-Advanced’6 as addressed by the ITU-R.7

A Universal Mobile Telecommunications System (UMTS) network system consists of three major sub-network parts; the UMTS Terrestrial Radio Access Network (UTRAN), the IP-based Core Network (CN), and the User Equipment (UE). The UTRAN consists of a set of radio network subsystems comprising a radio controller and one or more node base stations. The Base Station (BS) is referred to as Node-B, which can be collocated with a GSM Base Transceiver Station (BTS) and its control equipment to be called a Radio Network Controller (RNC). The RNC controls the handover decisions requiring signaling between the user equipment and the core switching network. The basic Core Network architecture for UMTS is based on the GSM network. All equipment in GSM systems has to be modified to upgrade to UMTS operation and services.

According to the UMTS Forum8, LTE is described as standardization work by the 3GPP to define a new high-speed radio access method for mobile communication systems. LTE is a continuing development project with the ultimate goal of achieving so-called fourth generation, or ‘4G,’ mobile wireless systems that will be an all IP-based broadband wireless packet network. Thus, LTE embraces the generic foundations of today’s 2G and 3G cellular systems addressed in GSM and CDMA (2G), GPRS, EDGE and CDMA2000-1xRTT (sometimes called 2.5G), and 3G systems such as UMTS, WCDMA, CDMA2000-EVDO, HSPA, HSPA+, and so on.

The immediate goal for LTE is an enhanced and optimized 3GPP Radio Access Network (RAN) called the Universal Terrestrial Radio Access (UTRA) network. The latest LTE specifications from the 3GPP are contained in Release 8 which was proposed to the ITU-R as a global standard and promised to be enhanced further to meet the eventual requirements of IMT-Advanced9. The core network of the LTE system consists of a flat, all IP-based simplified packet network architecture. In order to accommodate existing networks, new specifications for converting a variety of core network architectures are referred to as the Evolved Packet Core (EPC). Naturally, the EPC will be based on TCP/UDP/IP protocols that will be common to all wireless and fixed networks. While LTE tries to enhance the UTRA part of the 3G UMTS system by adopting a new air interface technology, its future version will also encompass all previous releases of 3G IMT-2000 systems. Thus LTE will be reverse compatible with the existing pre-released 3GPP family of networks as well as being capable of co-existing with 3GPP2-based networks such as CDMA2000.

The LTE air interface supports both frequency division duplexing (FDD) and time division duplexing (TDD) modes, each with a common 10 millisecond but different frame structure. The air interface also supports the multimedia broadcast and multicast service (MBMS) standard, a new technology for broadcasting content such as digital television in a point to multipoint fashion.

The LTE physical layer including an evolved-UTRA network defines a new radio access network based on OFDMA10 (Orthogonal Frequency Division Multiple Access) and IP packet-switched networks as specified in the aforementioned 3GPP Release 8. The radio access sub-system adopts an OFDMA-based access scheme for the downlink11 with constant subcarrier spacing of 15 kHz, dynamically variable modulation schemes, and other enhanced coding mechanisms to yield the variable data rates shown below. The smallest amount of resource that can be allocated in the uplink or the downlink is called a resource block (RB) and is 180 kHz wide. A resource block comprises minimally 12 subcarriers at the 15 kHz spacing. For the uplink, pre-coded OFDMA called Single Carrier - Frequency Division Multiple Access (SC-FDMA) is used.

The LTE channel is also scalable in terms of bandwidth and can range from 1.4 MHz to 20 MHz as contrasted with the current 5 MHz for the fixed UTRA channel. This is reflected in Figure 2 below along with the corresponding resource block configurations of each.

Figure 2. Transmission Bandwidth Configurations in LTE

The result is that the LTE scheme can be adapted to not only all existing IMT-200013 licensed bands in paired and unpaired spectrum allocations: 1.9~2 GHz, 850~900 MHz, AWS spectrum (1.7~2.1 GHz), but also other newly available spectrum bands at 2.3 GHz, 2.5 GHz, 3.4 GHz and possibly the new 700 MHz bands. Figure 3 below shows the current frequency band designations.

Figure 3. Frequency Bands Supported in LTE

It should be noted at this point that the mobile WiMAX14 standard also allows variable width spectrum bands and adopts the OFDMA access scheme. In certain spectrum bands, a possible competition in the commercial services market is foreseeable between mobile WiMAX operations and LTE. For a comparison, some of the technical details of Mobile WiMAX technology are described in our Tech Topic #1115.

The evolution of LTE is a completely new migration path for the air interface while network system compatibility is preserved. It should also be noted that advanced antenna technologies such as MIMO and beam forming are employed to further improve performance of the overall system with up to four antennas per base station sector. The concepts of multiple antennas are shown in the figure below. By providing diversity in transmitted signals, or diversity in received signals, the performance of the radio channel is enhanced.

Figure 4. Multiple Antenna Techniques

Additional capabilities of LTE will continuously evolve beyond the 3GPP Release 8 heading ‘LTE-Advanced’ to fulfill the future requirements specified by IMT-Advanced, also called 4G.

The trend of continuous increase in data traffic over the broadband Internet worldwide is obvious. Consequently, the need is apparent for new radio technologies with improved spectrum efficiency, higher average user throughputs, and simple packet-switch network architectures. Commercial mobile services for user applications are already moving toward all-IP based broadband wireless technologies that will reduce complexity and simplify the wireless core (flat network architecture) with lower cost.

By utilizing the commercial broadband operations, public safety users can acquire technical capability to access a wide range of mission related resources and services in addition to reducing the cost of deployment, while maintaining the QoS for critical voice applications. Thus, public safety communications may expect to implement broadband operations into its services in the future.

Currently, mobile service providers may consider advanced next generation wireless IP networks in the future among three major standardization initiatives:

1. 3GPP Long Term Evolution (LTE): GSM/UMTS-based technologies.

2. 3GPP2 Ultra Mobile Broadband (UMB): CDMA-based technologies.

3. WiMAX: IEEE 802.16 standard technology including Mobile version of 802.16e are also evolving, named ‘802.16m.’

Each network technology is under continuous development by respective concerned parties and is headed to the next generation of wireless service. All three of these technologies are converging to OFDMA radio access aiming for a common goal of achieving wireline medium performance wirelessly. Their architectures are similar in that they leverage a flat, all-IP network architecture requiring fewer nodes that enables service providers to integrate the core with the radio access network, providing real-time multimedia and broadband IP services from the core to the mobile user equipment. For service providers, deployment costs for a new service is an important factor to determine which to choose without disrupting the existing mobile services.

The simpler architecture will reduce latencies and thus optimize performance for real-time services such as voice and video. Voice application for public safety communications will remain critical. When conditions of QoS meet user requirements for dedicated applications, it is foreseeable under new network infrastructure that operating VoIP from mobile equipment of the emergency responder to the flat broadband IP network will be feasible. Perhaps, then, users in pre-allocated public safety narrowband spectrum may migrate to this advanced network infrastructure. After all, the advancement of commercial mobile technologies will contribute significantly to public safety communications using broadband services in the future.

Many of the commercial cellular carriers have announced their plans to move to LTE in the near future. Overarching to this development is the fact that most of the cellular carriers will implement some form of LTE, which gives a measure of commonality and interoperability built into NG mobile networks. Capitalizing on this commonality and the enhanced broadband capabilities should provide public safety entities and the first responder community with the requisite communications means to support all of their broadband applications in the future.

1 U.S. Department of Homeland Security, Statement of Requirements for Public Safety Wireless Communications and Interoperability, Version 1.1, Jan. 26, 2006.

10 OFDMA is an advanced multiple access mechanism using the Orthogonal Frequency Division Multiple (OFDM) technique to achieve high spectral efficiency and a great reduction of interference caused by various sources.
OFDM uses multiple sub-carriers like FDM but the sub-carriers are closely spaced to each other in an orthogonal manner without causing interference, allowing the removal of guard bands between adjacent sub-carriers.

Orthogonality between the sub-carriers means the peak of the spectrum of one sub-carrier coincides with the null of the spectrum of an adjacent sub-carrier. OFDM divides a very high-rate data stream or channel into multiple parallel low-rate data streams. Each low-rate data stream is then used to modulate a single sub-carrier to create a sub-channel. Depending on the distance from a base station, a sub-channel can be larger or smaller depending on shifting modulation schemes; including 64QAM, 16QAM, QPSK, or BPSK.

Numerous industry standard networking technologies in wireline, mobile or fixed wireless have adopted this OFDM-based access scheme such as ADSL, WiMedia UWB, Wi-Fi (802.11a/g/n), and WiMAX (fixed and mobile).
As an access method, OFDMA employs multiple orthogonally spaced sub-carriers like OFDM, but now the sub-carriers are divided into groups of sub-carriers redefining the sub-channel to consist of a group of sub-carriers. The sub-carriers that form a sub-channel can be arranged adjacently or be dynamically selected. A specific sub-channel can be assigned to each receiver in the downlink while a transmitter can be assigned one or more sub-channels in the uplink.

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